Solo-X has been replaced in GLM 4.2 by separate Solo and Mute functions, which together offer the same functionality as Solo-X. After selecting either Solo or Mute in the left-hand dropdown menu, a single mouse-click on any individual loudspeaker will solo or mute it while changing all other loudspeakers to the opposite state. Clicking the last-clicked loudspeaker a second time will revert it to its regular playback status. Clicking loudspeakers while holding the CTRL button down, or CMD button on Mac, will allow you to solo or mute multiple loudspeakers at once.